Perchlorate is an anion (ClO.sub.4.sup.-) which occurs in small amounts in nitrate deposits in Chile, in Chilean saltpeter, and in fertilizers imported into the US that are made using these nitrate deposits. In large quantities it is produced as ammonium perchlorate, which is manufactured as an oxidizer component in solid propellant for rockets, missiles, and fireworks. Because of its shelf life, it must be periodically washed out of the missile and rocket inventory and replaced with a fresh supply. Thus, large volumes of the compound have been disposed of in Nevada, California, Utah, and likely other states, since the 1950's. Ammonium perchlorate is also used in certain munitions, fireworks, the manufacture of matches, and in analytical chemistry.
Perchlorate has been found in many surface waters, such as Lake Mead, and in ground waters at concentrations ranging from 5 ppb (the detection limit) to 0.37%. The majority of locations where perchlorate has been detected in the groundwater are in California based on the presence of twelve facilities which have manufactured or tested solid rocket fuels for the Department of Defense or the National Aeronautics and Space Administration (NASA). Perchlorate has been found in 30% of the wells sampled in California, and is above the state's provisional action level in 9% of those wells.
Two facilities which manufactured ammonium perchlorate in Nevada have been found to have released perchlorate to groundwater which is the source for low levels (4-16 ppb) in Lake Mead and the Colorado River. This water is used for drinking water supply, irrigation and recreation for millions of people in Nevada, California, Arizona, and Native American Tribes. Perchlorate has also been detected in many other states, including Arkansas, Indiana, Iowa, Louisiana, Maryland, New York, Pennsylvania, Texas, Utah, Virginia, and West Virginia. The EPA expects perchlorate to be found in virtually all of the lower 48 states except perhaps Maine, Vermont, Connecticut, and Rhode Island.
Perchlorate exists in the environment as an anion from the solid salts of ammonium, potassium, or sodium perchlorate. Perchlorate salts are extremely soluble and stable when dissolved in water and are therefore not easily removed from water using conventional technologies. The perchlorate anion (ClO.sub.4.sup.-) is exceedingly mobile in aqueous systems and can persist for many decades under typical groundwater and surface water conditions, due to kinetic barriers to its reactivity with other available constituents.
Until recently, potassium perchlorate had been used therapeutically to treat hyperthyroidism resulting from an autoimmune condition know as Graves' disease. Potassium perchlorate is still used diagnostically to test thyroid hormones (TSH, T3 and T4) production in some clinical settings. The basis for the effect on thyroid hormone function is the competitive inhibition of iodide anion uptake by perchlorate which results in reduced thyroid hormone production. Thyroid hormone deficiencies lead to decreased metabolism, growth, and development. Perchlorate is believed to disrupt the production of normal levels of thyroid hormone in mammals by inhibiting the uptake of iodide anion by the thyroid gland.
Due to its interference with iodine in the production of hormones in the human thyroid, the contamination of waters with perchlorate is potentially a national concern. Perchlorate has recently been measured in several surface waters and groundwaters in the United States at concentrations up to 3700 mg/L. The California Department of Health Services (CDHS) based on the work by the U.S. Environmental Protection Agency (EPA), has established a provisional action level of 18 .mu.g/L for drinking water. The presence of perchlorate at these high concentrations in the aqueous environment, coupled with a very low drinking water standard, has created a national water contamination crisis in the United States potentially affecting 12 million people.
Typical water treatment technologies such as ion exchange, air stripping, carbon adsorption, and advanced oxidation are not currently cost efficient for the removal of perchlorate from drinking water. Perchlorate removal in membrane process, such as reverse osmosis, also has not yet been tested satisfactorily. In fact, a team of experts who recently met at a special workshop on perchlorate agreed that there is currently no proven removal process to treat perchlorate-contaminated water to drinking water standards. American Water Works Association Research Foundation (AWWARF). 1997. Final Report of the Perchlorate Research Issue Group Workshop, September 30 to October 2.
Despite the knowledge that perchlorate is biodegradable, no published reports on water treatment studies have demonstrated that perchlorate can be removed from contaminated waters to the levels required for drinking water while maintaining water quality sufficient to meet other drinking water primary and secondary standards.
Reactors have been used for purposes of wastewater treatment and drinking water treatment. To date, microbiological degradation of chlorate and perchlorate to low levels has been achieved only in engineered bioreactors suitable for wastewater treatment. The distinction between wastewater and water reactors is that for drinking water treatment, all toxic chemicals and most organic matter must be removed. For wastewater treatment, microbes and organic matter may remain in the water (subject to regulations) if it is to be discharged to a receiving water body or a publicly owned treatment works (POTW).
The development of biological reactors for water treatment requires new design strategies. No large-scale biological drinking water treatment systems currently exist in the United States. Thus, there is no established design basis for developing (per)chlorate drinking water treatment systems.
The present inventor has now discovered that perchlorate can be safely removed from drinking water sources through the use of an anaerobic biofilm packed bed system. In this system, perchlorate serves as an electron acceptor, and hydrogen gas or another oxidizable substrate serves as an electron donor. This unique system provides for the removal of perchlorate to below detectable levels.
It is therefore an objective of the present invention to provide a method and means of removing perchlorate from drinking water sources to safe and/or undetectable levels.
It is a further objective of the present invention to provide a method and means of removing perchlorate from drinking water sources wherein perchlorate is microbiologically reduced.
It is still a further objective of the present invention to provide a method and means of removing perchlorate from drinking water sources which limits subsequent biological growth in the water distribution system.
It is yet a further objective of the present invention to provide a method and means of removing perchlorate from drinking water sources which is economical.
These and other objectives will become apparent from the following description.